1. Field of the Invention
The present invention relates to an audio output apparatus of general information communication apparatuses, electro-acoustic apparatuses, measuring apparatuses and systems which deal with sound and more particularly to an antinoise or noiseproof type digital electro-acoustic transducer utilized in communication which transducer receives an audio signal from a digitized apparatus or system under noisy circumstances and produces sound.
2. Description of Related Art
Heretofore, when communication using sounds or speech sounds is made under noisy circumstances, there is a method in which a close-talking or noise-canceling type microphone is used in a transmission side. On the other hand, a noiseproof type receiver is often used in a receiving side as described in Architectural Acoustics Handbook, Gihodo, 1963. FIG. 5 shows a structure of such a receiver. In FIG. 5, numeral 11 denotes a soundproof housing, 12 a receiver body and 13 a pressure pad. Outside noise is insulated by the soundproof housing 11 to be reduced in level and reaches an ear.
However, the soundproof housing of the conventional noiseproof type receiver requires large thickness and heavy weight as a whole in order to obtain satisfactory sound insulation effect. In this case, it is troublesome to put on and operate the receiver. Further, when the receiver is put on, the pressure pad is put on the ear to cover the auricle. At this time, when the pressure exerted on the ear is increased, something wrong with the head is felt and an unpleasant feeling is given. Further, when the pressure is reduced, sound in a low frequency range, particularly, leaks from the housing and sufficient sound insulation effect cannot be obtained.
On the other hand, JP-A-10-126886 discloses a digital earphone in which electrodes of an electrostatic type electro-acoustic transducer are divided into 1:2:4:8: . . . and insulated to correspond to a plurality of bits of a digital signal and a digital microphone using the digital earphone. However, according to the literature, since one vibration plate is used in the earphone and the microphone in common, both electrodes of the earphone and the microphone must be disposed near the vibration plate, so that a structure thereof is complicated.
It is an object of the present invention to solve the above problems in the prior art by providing a noiseproof digital electro-acoustic transducer having excellent feeling of fitting and satisfactory sound insulation effect. Further, it is another object of the present invention to provide a digital electro-acoustic transducer which is simple in structure and easy to manufacture.
In order to achieve the above objects, the digital electro-acoustic transducer according to the present invention comprises a plurality of sound generating units for converting electrical signals into sounds, at least one sound receiving unit for producing an output signal in accordance with sound pressure exerted on a vibration film, a housing having a cavity in which the plurality of sound generating units and the at least one sound receiving unit are accommodated, driving means for driving the plurality of sound generating units on the basis of a drive signal, modulation means for sampling an output signal of the at least one sound receiving unit to produce a pulse in accordance with variation in amplitude of the output signal, and drive signal preparation means for calculating an arithmetic signal which reduces the amplitude of the output signal of the at least one sound receiving unit on the basis of the pulse supplied from the modulation means and superposing an externally supplied digital audio signal on the arithmetic signal to prepare the drive signal to be supplied to the driving means.